Thermal paper is used as labels for printing names and addresses of articles. For example, thermal labels may be adhered to plastic containers used in factories. Labels made of thermal paper are configured to develop color in response to heat. Therefore, characters and symbols can be written onto such labels with the use of a thermal head.
Recently, rewritable type thermal paper on which characters can be repeatedly written/erased, has been available in the market. When labels are used in logistics, it is convenient to be able to write/erase characters while the labels are attached to containers. Accordingly, there has been proposed a method of rendering characters by radiating laser beams onto the labels in a non-contact manner and generating heat (see, for example, patent document 1: Japanese Laid-Open Patent Application No. 2004-90026). Patent document 1 describes a relay lens system including plural lenses formed by flexible joints. An image formed by a laser light beam, which enters one end of the lens system, is transferred to (relayed to) the other end of the lens system.
The technology of forming images with the use of laser beams is conventionally known (see, for example, patent document 2: Japanese Laid-Open Patent Application No. 2004-341373). Patent document 2 describes an image forming method in which a single set of original image data is divided into plural lines, and a laser beam is radiated onto the photoconductive drum for each of the lines.
A typical laser beam has a diameter of at least approximately 0.3 mm, which is fairly large. Therefore, the stroke font is often used for rendering characters by laser beams. With the stroke font, there is no need to specify an outline and to fill in the area surrounded by the outline as in the case of the outline font, which is typically used for rendering characters. In the case of the stroke font, characters can be rendered as if they are written with a pencil, by scanning a laser beam according to coordinates defined by core data (data expressing the core) of a character. Furthermore, each line may be formed by a plurality of continuous lines. In this case, the connected lines are continuously rendered in such a manner as to trace the bent lines.
Meanwhile, there are cases of rendering thick lines on thermal paper with the use of a laser beam and the stroke font. Here, a thick line means a line that is thicker than a line width determined by the diameter of the laser beam. Thick lines are useful for highlighting characters and rendering barcodes. There are mainly three methods for rendering thick characters, as described below.                (1) Use outline font data or stroke font data to acquire outline data. Then, render the outline, and fill in the area inside the outline by raster scanning (see, for example, patent document 3: Japanese Laid-Open Patent Application No. S61-060177).        (2) Generate normal line vectors that are orthogonal to the strokes in the stroke data, without leaving any gaps between the normal lines. Then, render the normal line vector data to render thick lines (see, for example, patent document 4: Japanese Laid-Open Patent Application No. H08-279038, patent document 5: Japanese Laid-Open Patent Application No. H09-270018, and patent document 6: Japanese Laid-Open Patent Application No. H09-305779).        (3) Generate additional strokes parallel to an original stroke, without leaving any gaps between the strokes. Accordingly, the original stroke and the additional strokes appear to be a single line, and a thick line is generated (see, for example, patent document 7: Japanese Laid-Open Patent Application No. 2008-062506).        
However, the problem with the method (1) is that when the light beam has a large diameter, thick lines cannot be rendered unless the character is large. For example, when a light beam having a diameter of 0.3 mm is used to render a character of 5 mm2, the character may become blurred.
The same applies to the method (2). The generated strokes may have rough edges. FIG. 1 illustrates an example of one stroke rendered by the method (2). The laser beam has a substantially circular shape. Therefore, an uneven (bumpy) outline is formed by the end points of the plural normal lines, which are orthogonal to the stroke.
The method (3) does not have the same disadvantages as those of the methods (1) and (2). However, this method cannot be applied to characters with bent lines merely by generating additional strokes parallel to the original strokes without leaving any gaps between the strokes as described in patent document 7.
FIGS. 2A and 2B illustrate the problem of the method described in patent document 7. FIG. 2A illustrates an example of strokes included in a thick line, and FIG. 2B illustrates an example where the strokes have been rendered with a laser beam. In FIG. 2A, the thickness of the line corresponds to three strokes including two additional strokes parallel to the original stroke. The additional strokes are formed above and under the original stroke. The strokes form a line having a bent portion.
If the two additional strokes were generated by merely forming the same strokes as the original stroke at positions parallel to the original stroke, the strokes on the inner side of the bent portion would be too long, and the strokes on the outer side of the bent portion would be too short. Therefore, the strokes on the inner side have an overlapping part, and the strokes on the outer side have a detached part. The rendered character illustrated in FIG. 2B shows that excessive heat load is applied to the inner side of the bent portion where the laser beams overlap. Furthermore, on the outer side of the bent portion, part of the character is missing, which degrades the image quality.
In various characters, there may be strokes that intersect each other, or there may be strokes that bend at acute angles. In such cases, excessive heat load may be applied to an intersecting part because the radiated laser beams overlap each other. Furthermore, at a part where a stroke bends at an acute angle, the marking speed temporarily decreases, in consideration of the inertia of the device. As a result, the laser beam is radiated for a long time, and therefore excessive heat is applied.
FIG. 3 illustrates a Japanese katakana character “nu” (), which is rendered with two strokes. In this character, excessive heat load is applied to a part that bends at an acute angle 220, and also to an intersecting part 210.
Furthermore, when rendering characters with laser beams, the diameter of the laser beam needs to be taken into consideration.
FIGS. 4A and 4B illustrate examples of a Japanese katakana character “i” (), which is rendered with two strokes. In FIG. 4A, the diameter of the laser beam is sufficiently small, and therefore the two rendered strokes do not overlap. However, when the laser beam has a larger diameter, an overlapping part 230 is formed where the two strokes are close to each other, and therefore excessive heat load is applied to the overlapping part 230.
Accordingly, there is a need for an information processing apparatus, a laser radiation device, a render information generating method, a control system, a program, a recording medium, and a render information storage device capable of rendering thick lines with high image quality and without damaging the medium, even when the diameter of the laser beam is large with respect to the size of the diagram or character to be rendered.